1. Field of the Invention
The present invention relates generally to an apparatus for purifying an exhaust gas that is exhausted by an engine, and in particular to a technique contrived to detect a deterioration in a reducing catalytic converter incorporated in the exhaust gas purification apparatus, which employs ammonia or its precursor as reducing agent to reduce and purify nitrogen oxide (NOx) in exhaust gas.
2. Description of the Related Art
As catalyst purification system which eliminates NOx in engine exhaust gas, an exhaust gas purification apparatus disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-27627 has been proposed. In this conventional exhaust gas purification apparatus, liquid reducing agent is injection-supplied into exhaust gas moving on upstream of a reduction catalytic converter disposed in an engine exhaust gas system in compliance with an engine operating condition. As a result, catalyst reduction reaction between the NOx in the exhaust gas and the liquid reducing agent occurs, so that NOx is purified to become harmless components. As the liquid reducing agent, a urea aqueous solution is employed for inducing hydrolysis that generates ammonia through utilization of exhaust heat and water vapor in the exhaust gas.
A NOx purifying efficiency of the reduction catalytic converter is not semi permanently constant and is gradually lowered due to deterioration with lapse of time and the like. If the liquid reducing agent is injection-supplied into the exhaust gas in compliance with an engine operating condition at the time when the reduction catalytic converter is deteriorated, an amount of ammonia consumed by catalyst reduction reaction is reduced and therefore, the unconsumed ammonia might possibly be exhausted into the atmosphere. Further, when the reduction catalytic converter is deteriorated, the required NOx purifying efficiency may not be exerted, and thus the non-purified exhaust gas is possibly discharged into the atmosphere.